1. Field of the Invention
The invention proceeds from an apparatus for measuring the change of the intravascular blood volume as is known from EP 0 089 003.
2. Description of the Related Art
In blood purifying methods in which a fluid exchange or a withdrawal of fluid is provided it is necessary to control this liquid or fluid exchange in such a manner that undesired negative effects on the health of the patient are avoided.
Methods in which this necessity exists are for example hemodialysis, hemofiltration and plasma filtration.
The removal of fluid excess from the body of the patient requires a very precise control of the fluid balance and for this reason dialysis methods can only be carried out with fluid-balancing means. In spite of this precise balancing dialysis-typical unpleasant effects still occur in patients, such as headache, vomiting and muscular cramps. The probable reason for this is the too rapid extraction of sodium ions from the blood due to the concentration difference of sodium in the blood (extracorporeal circuit) and in the dialysis solution and in the too rapid fluid withdrawal.
It is known in hemodialysis to carry out the so-called volumetric ultrafiltration control. The prior art of ultrafiltration control or monitoring in hemofiltration is the balancing of the ultrafiltrate and the substitution solution with the aid of one or two weighing devices or balances, cf. for example DE-OS 3,132,790. These methods, which have already been industrially employed, permit a fluid extraction in accordance with the direction of the physician or operator, i.e. in accordance with the input over a predetermined period of time, a certain amount of fluid is withdrawn from the patient. It is also known to prescribe a so-called "ultrafiltration profile", i.e. a time-dependent variation of the ultrafiltration rate. The objective of the variation of the ultrafiltration rate with time is the withdrawal of the predetermined ultrafiltration amount from the patient in the way causing the least possible detrimental effects, i.e. in particular avoiding blood pressure drops.
In addition, by entering physiological and treatment parameters into such apparatuses for carrying out the ultrafiltration the change of the intracellular and extracellular volume can be predicted. Since these changes considerably influence the health of the patient the operator of the apparatus attempts to configure the ultrafiltration profile in such a manner that as uniform as possible a decrease of the extracellular volume takes place. Although this has led to an improvement of the otherwise unsatisfactory treatment result in the case of patients where the adjustment is difficult, this procedure is too complicated for a routine method.
Aforementioned EP 0 089 003 discloses a blood purification apparatus in which in the extracorporeal blood circuit a hematocrit measuring device is arranged which is connected to a control and evaluating unit. This hematocrit measuring device is based on an electrical resistance measurement of the blood during the blood filtration. From the change of the resistance values of the blood the change of the hematocrit is determined and from the latter the intravascular blood volume is deduced. Such resistance measurements have however the disadvantage that the measured values are falsified by other influencing factors such as flow rate, erythrocyte orientation, etc.
In DE-OS 3,640,089, which is based on the principle known from EP 0 089 003, from the values obtained from a conductivity relative measuring arrangement and from the conductivity of the fresh dialysis solution and the blood flow, as well as the performance parameters of the dialyzer, the plasma conductivity and the change of the plasma and blood conductivity are determined. Thereafter the hematocrit is calculated during the dialysis and from the hematocrit the change in the intravascular blood volume is deduced and in dependence upon the change of the intravascular blood volume the ultrafiltration rate determined.
A further apparatus for measuring the conductivity is known from EP-0029793.
Other methods for blood volume variation measurement are described for example in:
R. N. Greenwood, C. Aldridge, W. R. Cattell, Clinical Science (1984) 66, 575-583: "Serial blood water estimations and in-line blood viscometry: the continuous measurement of blood volume during dialysis procedures" PA0 and U. Schallenberg, S. Stiller, H. Mann, Life Support Systems (1987) 5, "A New Method of Continuous Haemoglobinometric Measurement of Blood Volume During Haemodialysis".
However of these known methods has so far led to industrial use because firstly there are difficult to surmount measuring and method problems and secondly complicated and additional apparatuses are necessary, such as conductivity measuring devices.